1. Field of the Invention
The present invention relates to a liquid crystal display device and, particularly, to such device in which the charge on a counter electrode is controlled not to vary.
2. Description of the Related Art
A liquid crystal display device displays an image by means of liquid crystal. The liquid crystal display device includes an upper glass substrate, a lower glass substrate, and a liquid crystal layer sandwiched between these substrates. Of the upper glass substrate, on its under surface facing the liquid crystal layer, a counter electrode for applying a common voltage Vcom to the liquid crystal layer and a transmission line X for supplying the common voltage Vcom to the counter electrode are situated. Of the lower glass substrate, on its upper surface facing the liquid crystal layer, a display electrode applying a display voltage to the liquid crystal layer and a transmission line Y for supplying a source voltage to the display electrode are situated.
In the arrangement as above, when a source voltage is applied to the display electrode and a common voltage Vcom is applied to the counter electrode through the transmission line X, a drive voltage determined by a potential difference between the applied source voltage and common voltage Vcom is applied to the liquid crystal layer.
The common voltage Vcom serves as a reference voltage for the voltage that is applied to the liquid crystal layer. For example, in a liquid crystal display device using an inversion driving method, with respect to the counter electrode, the polarity of charge supplied to the display electrode is inverted at given intervals. In this case, a drive voltage corresponding to a voltage difference between the display electrode and the common electrode in each interval is applied to the liquid crystal layer. For this reason, it is desired that the common voltage Vcom is stable for driving by the liquid crystal display device.
In the above arrangement of the liquid crystal display device, a common voltage Vcom that is applied to the counter electrode may become nonuniform. This is due to varying impedance of the counter electrode and varying wiring lengths of the transmission line through which the common voltage Vcom is supplied to the counter electrode. Nonuniform common voltage Vcom that is applied to the counter electrode results in nonuniformity in the drive voltage Vd per pixel applied to the liquid crystal layer and gives rise to a flicker in the screen and uneven image quality. One possible method for preventing an increase of transmission line impedance is to increase the wire diameter of the transmission line. However, this method is not practicable, because the larger the wire diameter, the smaller will be the aperture ratio of the glass substrate.
A technique concerning a common line wired on the glass substrate for transmitting a common signal is known.
Patent Document 1 (Japanese Published Unexamined Patent Application No. 2000-214431) discloses a semiconductor integrated circuit device having common output terminals and segment output terminals which output electric signals to drive a liquid crystal display panel, wherein the common output terminals are arranged virtually evenly at both opposite sides of the semiconductor integrated circuit.
According to Patent Document 2 (Japanese Published Unexamined Patent Application No. 2007-140384), in order to stabilize a common voltage Vcom, a supply voltage used as a reference for the common voltage Vcom that is applied to the counter electrode is supplied from a power supply circuit provided outside the liquid crystal panel.
The technique disclosed in the above Patent Document 1 provides even wiring lengths of the common line for transmitting a common signal. However, the impedance of the counter electrode is not uniform. There is still a possibility of failing to keep the common voltage Vcom applied to the counter electrode constant.
The technique disclosed in Patent Document 2 provides stable supply of the reference voltage for a common voltage Vcom. However, the wiring lengths of the transmission line are uneven and impedance differs from one portion to another of the counter electrode. Hence, there is still a possibility of failing to keep the common voltage Vcom applied to the counter electrode constant.